Apart from the dexterity required to play a theatre organ, there is the question of coordination in order to produce an appropriate combination of sounds for each musical item (or part thereof) from the available resources.

The chart (above) indicates the extent of the problem.  It shows that the complexity increases significantly as the size of the instrument increases.  Calculation of the number of permutations and combinations which can be engendered by a large number of independent functions would be a daunting task!  Each function, in reality, is merely an on/off switch.

All of these items are under the control of the organist (operator) who is producing an end result based on the combined use of a stop (rank at a footage) and a note in the scale. By introducing different combinations, at various stages throughout the piece being played, a musical “arrangement” results.

It should be noted at this point that any number of stops can be played (called a combination) at the same time by each note on the keyboard. However, the number of usable combinations is limited by the musicality (or lack of it) of the combination. The system is unaware of these limitations and the total number remains a possibility and is handled within the system.

To achieve the required result the original electrical switching systems (based on techniques borrowed from early telephone technology) were ingenious- and bulky. To the extent that they were not stowable within the console separate ‘relay’ rooms were required which were connected to the console and the chambers through massive intractable multicore cables- one function per strand of lacquered cotton wrapped wire.

In today’s world the majority of installations now rely on an electric/electronic control system as opposed to the electro-mechanical/pneumatic systems which persisted until the 1960’s when the electronic systems began to become available.  The interface (between player and console) elements are retained for ergonomic reasons. 

With the miniaturisation made possible by electronics the need for large relay rooms was eliminated. It became possible to place the systems required within the console, a small cabinet or simply a desk computer.  The connection between the console and the chambers is simplified to umbilical cables containing a minimum number of lines (typically one per rank plus control functions) as opposed to the previous heavy cables containing thousands of individual single function wires.  Interestingly, the number of wires required within the console and the chambers to connect each individual function to the system is still very large and in a typical installation can still require several kilometres of single core, plastic insulated wire.

The majority of theatre organs in the Melbourne area use a system supplied by Tonal Resources, Sydney (Malvern Town Hall, Kingston City Hall, Wesley of Warragul, Palace Dendy Theatre and Geelong College).  This system is based, for the most part on CMOS technology.  It is a simple multiplexed system scanning at 400 times per second.

Other installations (Her Majesty’s Theatre Ballarat, Coburg Town Hall) retain original systems.  The system used at the Regent Theatre is computer based.

PLAYING SYSTEM

Simply stated the electrical signals from stops and notes are combined within the relay and the resulting output is transmitted to and interpreted by the chamber electronics to activate each pipe of each rank as required.

While emphasis has been placed on the pipe control, it is to be remembered that there also several tuned percussions requiring similar levels of control. Un-tuned percussions and effects add to the mix and complexity.

In the chamber the signals received from the relay are distributed to rank drivers- one for each rank- from which individual pipes are activated.  Additional boards are required to control tuned percussions, untuned percussions (the “toy counter”), tremulants and shutters.  In the relay the output is divided into separate send boards for each chamber.  At Warragul there is only one chamber.

As shown in the diagram (below) pressing the middle C note on the keyboard and activating the 8 foot Tibia stop results in that one pipe being played.  Of course there are three keyboards and the pedal board- each division with a separate set of associated stops.  The combination is available in four different places each with the same result.  However, this may not be true for other ranks.  It depends on the planned specification for each instrument.

If the 4 foot Tibia stop is chosen our subject pipe will be played when the C below middle C is played.  But, then, that is heading into a separate complex subject know as unifying.

COMBINATION SYSTEM

Given the number of possible stop combinations and the dexterity required to set or change even a simple combination while playing (to say nothing of the mental effort to memorise the details of each combination) it becomes obvious that a recall system is required.

Below the front of each manual a row of push buttons (pistons) are provided so that a selection of combinations can be stored and recalled throughout the performance.

Originally an electro/mechanical system was provided in conjunction with the playing system. The problem was that it was not settable or changeable during a performance and was limited in its capacity. The system fitted to the Warragul Wurlitzer has the capacity to store 416 different combinations divided into eight channels.  One or more of these channels can be allocated, as required, to different organists.  This is particularly useful when more than one organist appears on the same programme.  The settings can also be retained over time until the organist returns for another performance.

Each stop driver board controls up to 64 stops.  Therefore, the Warragul installation requires two boards.

Neville Smith   Nov. 2014

HOW DOES IT WORK — WURLITZER CHEST ACTION

This description is intended to explain the working of the valve mechanism common to the majority of chests and pipes of a Wurlitzer organ. . The chest design has never been bettered and is still used today without modification. That is not to say that there aren’t other designs as used by other companies. Wurlitzer has become a generic term used to classify theatre organs. (cf: Compton , Christie, Morton, etc.).  

A Wurlitzer chest usually carries 61 pipes (or five octaves) of a particular rank set in a double row. These pipes are not set chromatically from one end of the chest to the other. Rather, the lowest notes are at each end of the chest- C at one end and C# at the other with notes ascending to the middle of the chest. (C,D,E,F#,G#,A#,C and so on- C#,D#,F,G,A,B,C# and so on-). Each pipe requires a set of valves to operate. The mechanism is repeated for each pipe along the length of the chest. Therefore it requires compact nesting to achieve a satisfactory outcome. In turn chests can be nested to support up to nine ranks. The diagram (below) shows two chests.

Chests are made entirely of wood and the valve system relies on galleries within the chest walls to transmit air pressure. As the galleries extend across various mating surfaces extensive gasketing is required.

The complexity of the structure is indicated in the actual arrangement drawing. To understand the operation of the system an exploded diagram is shown. In this view all parts of the system are at rest i.e. no sound. Although all parts are under the influence of the internal chest air pressure (usually from 10” to 15” WG) there is no air flow or loss of pressure from within the chest.

In essence the object of the system is to overcome the influence of the pallet spring and internal wind pressure which otherwise keep the pallet closed.

When a note is required to be sounded a series of actions occur- with the understanding that the elapsed time frame is almost instantaneous. (A criticism of the system is that it does not allow the organist any control over the attack and decay of the note- which is a claimed feature of the classical tracker action).

1.       A current is applied to the electromagnet which attracts the armature off its seating.  This closes the access to chest air and opens the gallery to atmosphere.  

2.       The primary motor (bellows) collapses under the influence of the chest pressure thus lifting the primary valve.

3.       When the primary valve rises it closes access to chest air and opens the next gallery to atmosphere.

4.       The secondary motor (bellows) collapses under the influence of chest pressure. The size of the motor is such that there is sufficient force generated to overcome the chest pressure on the pallet and the strength of the pallet spring.

5.       The pallet opens and chest air flows to the pipe.

The pipe will continue to sound until the current is removed from the magnet.

The sequence which follows is equally instantaneous:

6.       The armature drops back to its seating and chest pressure resumes in the gallery.

7.       With equal pressure inside and outside the primary motor the valve drops under the influence of gravity and chest pressure. In its lowered position chest pressure is restored in the next gallery.

8.       With no pressure differential in the secondary motor the pallet spring is able to close the pallet with the assistance of the chest pressure on the underside of the pallet. The action is restored to rest position.

Easy!  Now multiply the action by, say 40 times- multiple notes (chords) from multiple ranks played on two manuals (or three if they are available and dexterity allows) and the pedal board at the same time- all responding at the same speed.

Wind to operate the chosen pipes is supplied at the required pressure for each rank from the regulators.

There are variations in the actual layout of the chest depending on the size of the pipes supported. There are chests which do not play pipes but which through mechanical linkage play percussion instruments- either tuned (such as xylophone or chrysoglott) or untuned (drums, cymbals).  In the majority of these the secondary motor is connected directly to a playing hammer which taps, or hits, the note required rather than activating a pallet.  Additionally there are effects such as whistles, sirens, horns and bells each requiring individual adaptations of the basic mechanism to operate.

Neville Smith   Nov. 2014